1. Field of the Invention
The present invention relates to an electromagnetic pump, and more particularly, to an electromagnetic pump suitable for use in a separating oil pump of a two-cycle engine.
2. Background Art
An electromagnetic pump--which applies a pulse current to a solenoid, to thereby reciprocate a plunger and draw oil in and force oil out--has conventionally been employed as a separate oil pump of a two-cycle engine. FIG. 3 shows an example of such a conventional electromagnetic pump. An illustrated plunger 2 is slidably fitted into a cylinder 1 formed from non-magnetic material, such as aluminum or brass, and is forced rightward by means of restoration force of a compression coil spring 8.
An inner yoke 3 opposite the plunger 2 is pressingly fitted into an end yoke 4, and the end yoke 4 is fixed to an outer yoke 5 by means of caulking. A nipple 6 presssingly-fitted into the outer yoke 5 is in close proximity to the plunger 2. The plunger 2, the inner yoke 3, the end yoke 4, the outer yoke 5, and the nipple 6 are formed from magnetic material and constitute a magnetic circuit.
A coil 16, which is wound around a plastic coil bobbin 13 and applies magnetomotive force to the magnetic circuit, is covered with the coil bobbin 13 and a plastic mold 14. The plastic mold 14 is housed in the space which surrounds the cylinder 1 and which is defined by the end yoke 4 and the outer yoke 5. The edge of the outer yoke 5 is locked to the end yoke 4 by means of caulking, while the plastic mold 14 remains in pressing contact with the end yoke 4 by means of a cushion member 23 interposed between the outer yoke 5 and the plastic mold 14. Power is supplied to the coil 16 from an electrode 15 embedded in the plastic mold 14.
A valve seat 7 is pressed into the inner yoke 3 while being properly positioned. A discharge valve 12 is forced by the compression coil spring 10 so as to close a flow channel of the valve seat 7. A valve seat 24 is pressingly fitted into the plunger 2. An inlet valve 11 is forced by a compression coil spring 9 so as to close a flow channel of the valve seat 24.
An O-ring 19 hermetically seals a space between the inner yoke 3 and the cylinder 1, and an O-ring 17 hermetically seals a space between the nipple 6 and the cylinder 1. A spacer 20 interposed between the nipple 6 and the plunger 2 controls the maximum magnetic gap between the plunger 2 and the inner yoke 3; i.e., a plunger stroke.
In the electromagnetic pump having the foregoing configuration, when an electric current flows through the coil 16, a magnetic field develops in the magnetic gap between the plunger 2 and the inner yoke 3, as a result of which the plunger 2 is attracted by the inner yoke 3 against the restoration force of the compression coil spring 8. When the electric current flowing through the coil 16 is shut off, the plunger 2 is separated from the inner yoke 3 and is brought into pressing contact with the spacer 20, by means of restoration force of the compression coil spring 8.
The plunger 2 reciprocates in the manner as mentioned previously. When the plunger 2 is moved rightward the discharge valve 12 is closed and the inlet valve 11 is opened, whereby oil is drawn into a pump chamber (a space between the discharge valve 12 and the inlet valve 11) from the nipple 610 and the center hole of the plunger 2, by way of a gap between the inlet valve 11 and the valve seat 24. In contrast, when the plunger 2 is moved leftward, the discharge valve 12 is opened and the inlet valve 11 is closed, whereby oil is forced out to an oil flow channel of the inner yoke 4 from the pump chamber, by way of the space between the discharge valve 12 and the valve seat 7. An engine control unit controls a pulse current which is to be applied to the coil 16 in response to a signal output from a sensor for detecting the working state of the engine, thus controlling the amount of engine oil to be supplied.
The flow rate of the electromagnetic pump is determined from the number of pulses of the electric current and plunger strokes. The stroke of the plunger 2 corresponds to a difference between the distance between the end face of the spacer 20 and the end face of the inner yoke 3 and the distance between the end face of the plunger 2 and a plane at an opposite end portion of the plunger 2, which contacts the end face of the spacer 20. Tolerances of many parts contribute to the distance between the end face of the spacer 20 and the end face of the inner yoke 3.
More specifically, tolerances stemming from the pressing of the nipple 6 into the outer yoke 5, fixing of the end yoke 4 to the outer yoke 5 by caulking, and dimensional tolerances of the nipple 6, the cylinder 1, the plunger 2, the spacer 20, and the inner yoke 5, contribute to the distance.
In terms of electrical conditions under which the plunger 2 can be actuated, the diameter of the plunger is limited to a value of 6 mm to 7 mm. If the diameter of the plunger is made smaller than this range, the plunger cannot be actuated. Further, in order to diminish power consumption, the stroke of the plunger 2 must be made smaller. In consideration of the amount of oil required to be delivered, the stroke of the plunger 2 assumes a value of 0.5 mm or less for a two-cycle engine. If the tolerance of flow rate is reduced to 10% or less, variations in the stroke of the plunger 2 must be held to .+-.0.05 mm or less. Thus, in order to reduce the tolerance of stroke of the plunger, the spacer 20 must be prepared in various sizes, and adjustment of stroke requires a lot of time.
Further, since the compression coil spring 9 for constraining the inlet valve 11 is disposed within the pump chamber, the dead volume of the pump chamber becomes large, thereby resulting in a decrease in compression ratio and a drop in air displacement capability. If the air displacement capability of the pump is too small, in the worst case the pump fails to supply oil because of an air-lock phenomenon.
The present invention has been conceived in view of the foregoing problems of the prior art, and an object of the present invention is to provide an electromagnetic pump capable of readily and precisely determining the stroke of a plunger. Another object of the present invention is to provide an electromagnetic pump having large air displacement capability.